MANILA, Philippines (AP) — The Philippines has expressed concern to China over an increasing number of Chinese radio messages warning Philippine aircraft and ships to stay away from newly fortified islands and other territories in the South China Sea claimed by both countries, officials said Monday.

A Philippine government report seen by The Associated Press showed that in the second half of last year, Philippine military aircraft received Chinese radio warnings at least 46 times while patrolling near artificial islands built by China in the South China Sea’s Spratly archipelago.

The Chinese radio messages were “meant to step up their tactics to our pilots conducting maritime air surveillance in the West Philippine Sea,” the report said, using the Philippine name for the South China Sea.

Philippine officials have raised their concern twice over the radio transmissions, including in a meeting with Chinese counterparts in Manila earlier this year that focused on the Asian countries’ long-unresolved territorial disputes, according to two officials who spoke on condition of anonymity because they were not authorized to discuss the issue publicly.

It’s a new problem that emerged after China transformed seven disputed reefs into islands using dredged sand in the Spratlys, where the new islands now stand in close proximity to islands occupied by Vietnam, the Philippines and Taiwan. Malaysia and Brunei are the other claimants to the chain of islands and barren islets and atolls.

The messages used to originate from Chinese coast guard ships in past years but military officials suspect transmissions now are also being sent from the Beijing-held artificial islands, where far more powerful communications and surveillance equipment has been installed along with weapons such as surface-to-air missiles.

“Our ships and aircraft have observed an increase in radio queries that appear to originate from new land-based facilities in the South China Sea,” Commander Clay Doss, public affairs officer of the U.S. 7th Fleet, told the AP by email in response to questions about the Chinese messages.

“These communications do not affect our operations,” Doss said.

Although the U.S. lays no claims to the strategic waterway, its Navy has deployed ships and aircraft in operations to promote freedom of navigation and overflight, but which China protests as foreign meddling in an Asian dispute.

U.S. Navy ships and aircraft communicate routinely with regional navies, including the Chinese navy. “The vast majority of these communications are professional, and when that is not the case, those issues are addressed by appropriate diplomatic and military channels,” Doss said.

A Philippine air force plane on patrol near the Chinese-held islands received a particularly offensive radio message in late January when it was warned by Chinese forces that it was “endangering the security of the Chinese reef. Leave immediately and keep off to avoid misunderstanding,” according to the Philippine government report.

Shortly afterward, the plane received a veiled threat: “Philippine military aircraft, I am warning you again, leave immediately or you will pay the possible consequences.” The Filipino pilot later “sighted two flare warning signals from the reef,” said the report, which identified the Chinese-occupied island as Gaven Reef.

Chinese Embassy officials did not reply to requests for comment, but Beijing officials have repeatedly said they have the right to build on what they say is their territory and defend their sovereignty at all costs.

Philippine air force chief Lt. Gen. Galileo Gerard Rio Kintanar Jr. said Filipino pilots respond calmly to the Chinese radio messages and proceed with their mission as planned, adding that the higher number of reported radio challenges reflects the Philippine military’s commitment to protect the country’s territorial interests through intensified patrols.

“They do that because of their claim to that area and we have a standard response and proceed with what we’re doing,” Kintanar said.

In April last year, Chinese forces attempted by radio to drive away two Philippine military aircraft carrying the country’s defense and military chiefs, along with other top security officials and about 40 journalists, to Philippine-occupied Thitu island, which in the Philippines is called Pag-asa.

Thitu is more than 22 kilometers (14 miles) from Subi Reef, once a coral outcrop that now resembles a small Chinese city with a runway and skyline visible from the Philippine-held island when its lights glow at night.

China warned the Philippine planes that they were straying into its territory, and that they should turn around to avoid any mishap. The aircraft responded by saying they were flying over Philippine territory.